The invention concerns a nuclear magnetic resonance apparatus for generating a homogeneous, static magnetic field in the z-direction, comprising at least one coil/resonator system for transmitting and/or receiving radio frequency (RF) signals at at least one measuring frequency, a gradient system for generating pulsed field gradients in at least one spatial direction, and a shielding configuration which is positioned radially between the at least one coil/resonator system and the gradient system, wherein the shielding configuration comprises at least one electrically conducting layer with at least one continuous slot, wherein the electrically conducting layer is disposed about the center of the shielding configuration axially symmetrically with respect to the z-axis.
A nuclear magnetic resonance apparatus of this type is disclosed e.g. in [9].
A gradient shielding configuration should be impenetrable for the radio-frequency fields of the coil/resonator system, but should not perturb switching of the (DC) gradient pulses, which are within a frequency range of an order of magnitude of ≦10 kHz.
In order to achieve this, conventional shielding configurations have slots, which interrupt the current paths the eddy currents induced during switching of the gradients “want” to take. At the same time, the mirror currents, which permit shielding of the RF fields, must still be allowed to flow. This is obtained through bridging the slots with capacitive elements which appear permeable to RF currents but which block the quasi DC currents during switching of the gradients.
There are currently two solutions for configuration of the slots:
In the conventional devices of [4],[5,],[7],[10], an electrically conducting, metallic layer is structured by annular conductor elements in such a manner that the mirror currents of the RF current are “imitated”. Each of these annular conductor elements is additionally cut again to stop the eddy currents from the gradients. In order to create the RF path, a second layer is separately disposed using a dielectric to produce a capacitive connection for the RF shielding currents. Alternatively, a discrete capacitor is disposed across the slots, which closes the path for the RF currents.
Another approach suggests n-fold division in an axial direction, wherein overlaps and/or installation of discrete capacitive elements provide sufficient capacitive couplings between the stripe-shaped sections [1]-[3],[6],[8],[9]. Additional slots may be provided in the radial direction. In some embodiments, not all slots completely intersect the conducting layer in the z-direction.
Positioning of the slots, in particular, in the region of the coil/resonator system is generally influenced by the RF conditions. On the other hand, additional slots may be provided at certain locations to further attenuate the remaining eddy currents.
There are two simplified models to illustrate the effect of the slots.
The first model assumes that, despite the slots, the eddy currents try to continue to flow like in a shielding configuration without slots (FIG. 19a). FIG. 19b shows that in the slotted shielding configuration (shown in a developed view) additional currents only flow along the perpendicular slots. This increases the resistance for the shielding currents, thereby reducing the decay times. The larger the amount of slots introduced, the faster the decay of the eddy currents. FIG. 19c also shows that a radial slot in the center is helpful, in particular, for a z gradient, since it separates the regions with left-turning currents from those with right-turning currents.
A second model is based on the assumption that the flux remains undisturbed and the currents in the individual sections are distributed in accordance with the fluxes flowing through them. It is thereby assumed that a current is finally interrupted when it has been stopped by a slot (i.e. only one single axial slot is required and the rest are radial slots, as is shown for a z gradient in FIG. 20b).
In contrast to the first model (FIG. 19b), wherein the region of the highest currents (and at the same time lowest perpendicular flux) requires the greatest amount of slots, the second model (FIG. 20b) requires the finest division in the end region (outside of the reversal points of the gradient) and in the center, since the major part of the flux must “diffuse through the slots” at this location. FIG. 20a shows the perpendicular magnetic flux in dependence on the axial coordinate z.
In fact, both models do not adequately describe the real situation. They merely represent the two limiting viewpoints (undisturbed current in the first case and undisturbed flux in the second case). In order to obtain more precise information about the ideal position of the slots, a simulation calculation is required which can show the temporal decay of the gradient fields after switching off for a concrete case. The slots can then be positioned in an iterative manner. One problem thereby is, however, that the couplings to the resonator systems, possibly shim coils, tubes, elements of the resonator system etc. must also be taken into consideration.
The shielding effect of the shielding configuration relative to the RF fields in both above-mentioned conventional embodiments is based on the fact that the first eigenresonance of the RF shielding configuration is below the resonance frequency of the NMR coil. If this were not the case, the capacitive coupling would be insufficient to shield the RF fields.
The thickness of the electrically conducting layer is selected in each case to minimize the RF loss, and at the same time maximize the DC resistance. Layer thicknesses of approximately 3 skin depths (for NMR frequency) are currently used.
In particular, in the shielding configurations disclosed in [1]-[3],[6],[8],[9], which basically represent a radial configuration of metal strips, a plurality of eigenresonances are produced on the shielding configuration which can cause problems. In principle, such a configuration has as many modes as strips. FIG. 21 shows this by way of example for a configuration with four strips. An additional spectrum of higher harmonics is obtained when the wavelength decreases compared to the dimensions. The higher the number of elements used in such a shielding configuration, the more resonances it contains. In order to realize a maximum transparency for the gradient fields, the subdivision must be very fine (e.g. n=8, 16, 32). In this case, the spectrum becomes so dense that only very few frequency bands remain for the measuring frequencies.
Such gradient shieldings are generally used in MRI, which comprises one or maximally two measuring frequencies, with both frequencies being generally considerably below 200 MHz. In this case, the shielding configuration can be designed through selection of the capacitances between the strips, such that none of the maximally two measuring frequencies collides with the eigenmodes of the shielding configuration. If this were the case, the shielding effect would be considerably reduced and the Q-value and/or efficiency of the resonator system would be deteriorated.
For NMR systems, the situation is different. There are generally at least four measuring frequencies, possibly even a very wide band of frequencies that must all be tunable. It is basically impossible to design shielding configurations consisting of several elements in such a manner that none of the measuring frequencies comes close to a eigenresonance of the shielding configuration, while at the same time ensuring sufficient shielding effect for all modes.
The documents [11] and [12] disclose a coil type which geometrically cancels the coupling between two coils through geometric arrangement of the conductors being rotated by n*2π with respect to the window of a further coil. In this case, the integral of the magnetic flux of one coil that interacts with the other coil is zero.
The purpose of the invention is to propose a nuclear magnetic resonance apparatus with a shielding configuration that is designed in such a manner that only little or no energy is coupled from the resonator/coil system into the shielding configuration.